MEMS devices, such as pressure sensors, accelerometers, gyroscopes, and the like, typically comprise a sensor structure having a characteristic that can vary and that is altered in a predictable manner through interaction with an external factor, such as heat, humidity, pressure, movement, and the like. The sensor structure is configured to act as part of a transduction system for the MEMS device which uses the varying characteristic of the sensor structure to generate an electrical signal having a value is indicative of the external factor.
MEMS devices are typically encapsulated in some manner to protect and isolate the device from contamination from undesired environmental factors, such as heat, humidity, dust, and the like, which could adversely impact device performance. The encapsulation often requires hermetic seals to protect the ambient environment in order to ensure optimal operation. Some MEMS devices also require a specific vacuum ambient for optimal operation. For example, MEMS bolometers and gyroscopes require high vacuum, e.g., ˜10 μbar, while MEMS accelerometers require low vacuum, ˜10 mbar.
The different vacuum requirements of different devices has made it difficult to achieve optimal vacuum level requirements for different MEMS devices incorporated onto the same chip. FIG. 1 depicts a device 10 that includes a MEMS wafer 12 on which a MEMS accelerometer 14 and a MEMS gyroscope 16 have been fabricated. A cap wafer 18 is bonded onto the MEMS wafer 12 which defines cavities 20, 22 over both the MEMS accelerometer 14 and the MEMS gyroscope 16 which are separated by a dividing wall 24. Current MEMS packaging typically happens at a wafer level, which results in all the devices typically sealed at the same vacuum levels. ‘Getter’ materials are typically used to locally evacuate the sealed cavities 20, 22 to achieve the desired vacuum level for certain devices, such as the gyroscope 16 of FIG. 1. A getter material is typically some alloy material capable of absorbing air molecules (N2, O2, H2 etc.,) into its interstitial spaces thereby evacuating the sealed ambient to high vacuum levels. Getter materials may not be capable of achieving the required vacuum ambient for different MEMS devices when starting from the same sealed vacuum level.